Bone resorption by osteoclasts is essential for normal bone development. Osteoclast deficiency leads to osteopetrosis, which is characterized by increased bone mass and may lead to bone deformities or in severe cases, to death. Increased numbers and activity of osteoclasts, on the other hand, cause increased bone resorption, and may lead to osteoporosis and other osteolytic diseases. A better understanding of the molecular regulation of osteoclast formation, activity, and survival will provide novel targets for therapeutic intervention in the control of these diseases. We recently identified and cloned a gene, NHAoc/NHA2, that encodes a novel Na+/H+ antiporter that is the first mitochondrial NHA characterized to date. NHAoc/NHA2 is highly and selectively expressed in osteoclasts and displays the expected ion transport activities of a bona fide NHA. We have demonstrated that this gene plays a role(s) in normal osteoclast differentiation, apoptosis and bone resorptive function in vitro. Extensive mutational analysis of a bacterial homologue, NhaA, has revealed a number of amino acid residues essential for its activity. Some of these residues are evolutionarily conserved and have been shown to be essential not only for activity of NhaA in bacteria, but also of NHAoc in eukaryotes. We hypothesize that evolutionarily conserved amino acids that are essential for NhaA antiporter activity, pH regulation and dimerization will have a similar role in NHAoc/NHA2 and that mutations in those amino acid residues will impact NHAoc activity and therefore osteoclast function in vitro and in vivo. PUBLIC HEALTH RELEVANCE: Osteoclasts are cells that are responsible for bone removal ('resorption') during normal bone development and maintenance. Abnormal osteoclast numbers and/or activity, on the other hand, can cause a spectrum of diseases ranging from osteopetrosis to osteoporosis. This project seeks to determine the role of a novel gene that we have discovered in osteoclasts, termed 'NHA-oc/NHA2', in regulating bone mass. This work will aid us in the design of appropriate new therapies based on drugs that interfere with NHA-oc/NHA2 activity for the prevention of pathological bone loss.